Aid agencies are scrambling to try to get water and food to people in the Philippines who've been left homeless or injured by Typhoon Haiyan.

But reaching some of the areas ravaged by the intense storm is proving difficult. Even when aid can make it onto the islands, it's still not clear what supplies are needed the most.

An estimated 10,000 people are presumed dead and more than 600,000 people have been left homeless by the Category 5 typhoon, the United Nations said Monday.

One of the hardest hit areas is the city of Tacloban. The airport outside the city of about 220,000 has finally opened up, but there's so much debris in the roads that it's hard for relief workers to even leave the airport.

"The route from the airport to the city itself is only 11 kilometers (6.8 miles)," John Ging, with the U.N. Office for the Coordination of Humanitarian Affairs, said at a press briefing Monday in Geneva. "But it takes a six-hour round-trip journey ... to get from the airport to the city again because of the devastation."

Typhoon Haiyan, which is known as Yolanda in the Philippines, came ashore with sustained winds of almost 150 mph. There are also reports of a devastating storm surge that pushed a huge wall of water inland.

Many places are strewn with dead bodies, Ging says. And the first challenge is to bury the bodies. "But it's not just that," he says. "It's also [providing] clean drinking water for those who are alive. Food is also a big issue.

"Entire areas have been completely and utterly decimated." Ging says. "So keeping [the survivors] alive, as well as dealing with the corpses they're concurrent priorities."

Various U.N. agencies are sending in emergency teams with aid supplies. Private humanitarian groups also are mounting the largest relief operation since the Haitian earthquake in 2010.

The nonprofit Doctors Without Borders is shipping 350 tons of tents, generators, medical supplies and other gear into Manila on chartered planes from warehouses in Brussels and Dubai.

The biggest challenge right now is trying to assess exactly what the needs are in the parts of the islands hit hardest, says Henry Gray, an emergency coordinator with Doctors Without Borders.

"On huge parts of the island chain, we just don't know what's out there," Gray says. "We know that there are millions of people in the affected zone. We know that there are massive needs. But getting a clear picture is really difficult.

There are reports from some parts of the island of Cebu that 90 to 95 percent of the buildings have been destroyed. Hospitals and clinics were flattened.

Doctors Without Borders is assuming there are a wide variety of medical needs, Gray says. "There will be people with traumatic injuries from flying debris, falling trees, crashing walls," he says. "Then there will be the next phase ... in which people who are not necessarily badly wounded ... succumb because of the unsanitary conditions."

In the coming days, the focus of the relief effort will be to make sure people in the areas hit hardest have access to the bare basics: clean drinking water, sanitation and a roof or tarp to sleep under. The wounded need to be sewn up. The dead need to be buried. Roads need to be cleared.

Then, eventually, agencies and communities can start to think about rebuilding homes, schools and communication towers.

This week is just the beginning, Gray says. "There is a long game here. Getting this part of the Philippines back on its feet is going to take an incredible effort."